In the field of healthcare, fitness and leisure activities, the use of activity trackers or activity monitors for monitoring a physical activity of a subject has become more and more popular. Starting from pedometers, which count the number of steps, the technology has evolved towards more sophisticated activity trackers which distinguish between different types of physical activities such as walking, running or cycling. For example, walking and running can be distinguished from being sedentary based on a step frequency. Exemplary systems feature for example a wrist band with a sensor, wherein the wristband is in communication with a smart phone that is running a fitness tracking app.